It is known to pre-separate ions temporally according to their ion mobility using an ion mobility spectrometer or separator (“IMS”) prior to mass analysing the ions using a mass analyser. The mass analyser is arranged to mass analyse sequential packets of ions which emerge from the ion mobility spectrometer or separator and wherein each packet of ions comprises ions having substantially the same ion mobility.
The known ion mobility spectrometer or separator comprises a plurality of ring electrodes and a constant electric field is maintained along the entire length of the ion mobility spectrometer or separator as the distribution of ions progresses along the length of the ion mobility spectrometer or separator. A pressurised static buffer gas is provided within the ion mobility spectrometer or separator.
Analyte ions are pulsed into the ion mobility spectrometer or separator and the ions are then accelerated axially by the electric field. The analyte ions experience repeated collisions with the buffer gas molecules as the ions travel axially along the length of the ion mobility spectrometer or separator.
The analyte ions reach a constant drift velocity in the drift direction after continuous accelerations and collisions. The constant electric field exerts a greater force on higher-charge ions whilst larger ions experience more collisions with the buffer gas. As a result, the analyte ions are separated according to their ion mobility. It will be understood that the mobility of an ion is a function of the collisional cross-section of the ion with the buffer gas and also the charge of the ion.
A particular problem with known ion mobility spectrometers or separators is that in order to achieve a relatively high degree of temporal separation (i.e. to achieve a relatively high degree of separation according to the mobility of the ions) it is necessary for the ion mobility spectrometer or separator to have a relatively long drift length. However, constructing an ion mobility spectrometer or separator having a relatively long drift length is problematic since it necessitates the use of relatively high voltages which must be applied to at least the initial stages of the ion mobility spectrometer or separator.
It will be understood that applying relatively high voltages to at least some sections of the ion mobility spectrometer or separator requires the provision of relatively expensive and complex high voltage circuitry which is problematic.
Another disadvantage of applying high voltages to sections of the ion mobility spectrometer or separator is that the high voltages are likely to cause discharge or arcing if grounded objects are located nearby. As a result, the use of high voltages with an ion mobility spectrometer or separator imposes significant design constraints and necessitates the use of relatively complex and expensive electronics.
A yet further problem with an ion mobility spectrometer or separator which utilises high voltages is that the power consumption can also be relatively high.
WO 2013/093513 (Micromass) discloses an ion mobility separation device comprising an ion guiding path that extends in a closed loop. A DC voltage gradient is maintained along at least a portion of the longitudinal axis of the ion guide.
US 2013/292562 (Clemmer) discloses an ion mobility spectrometer in which a drift tube is partitioned into plural drift tube segments. Voltage sources V1 and V2 are connected to pairs of segments, and are alternately switched on to alternately establish two different constant electric fields across pairs of drift tube segments.
It is desired to provide an improved ion mobility spectrometer or separator and an improved method of separating ions according to their ion mobility.